The present invention, in some embodiments thereof, relates to photodetection and, more particularly, but not exclusively, to the detection of photons by an active region which comprises bandgap modifying atoms.
Recording and measuring a weak signal presents challenging and acute problems for the designers of modern sensors for myriad applications in diverse fields of science and technology. In these sensors, various primary signals (optical, ultrasonic, mechanical, chemical, radiation, etc.) are transformed into elementary charge carriers, such as electrons, holes or ions. Signal charge packets of such elementary charge carriers are amplified and converted to an electrical signal which is fed into a recording or analyzing device and/or used as a feedback signal for monitoring.
One approach to the detection of weak optical signals is the use of photodetectors in which the exposure times are long. These photodetectors typically employ semiconductor technology. Since the energy of photon is inversely proportional to its wavelength the detection of long wavelength photons, particularly in the infrared (IR) range, is more difficult.
Currently, prevalent infrared photodetection technology is based on interband (IB) absorption, wherein (IB) transitions occur in narrow bandgap semiconductors such as HeCdTe, InSb, InAs and InGaAs. Another technology is based on intersubband (ISB) transitions in heterostructures in a configuration known as Quantum Well Infrared Photodetectors (QWIP), wherein the photodetection mechanism is via absorption between subbands rather than between the valence and conduction bands. An additional technology is based on type-II superlattice structures engineered by deposition of a stack of successive semiconductor layers. Although much effort is invested in improving the performances of all types of IR detectors, none of the above technologies was proven to be sensitive enough for single photon detection.
Another class of detectors is based on a unipolar barrier layer between a contact layer and an absorption layer. The barrier layer suppresses a dark current and allows the device to operate with low diffusion current. The barrier is designed so that it only blocks the passage of majority carriers, while the minority carriers are free to flow from the absorbing layer to the contact layer. The term “unipolar barrier” was coined recently to describe a barrier that can block one carrier type (electron or hole) but allows the un-impeded flow of the other. The barrier can also prevent the depletion of the narrow bandgap photon absorbing layer suppression of generation-recombination (G-R) dark current due to Shockley-Read-Hall (SRH) processes. Such devices have been termed nBn [Maimon et al. “nBn detector, an infrared detector with reduced dark current and higher operating temperature,” Applied Physics Letters 89, 151109 (2006)] or, more generally, XBn [P Klipstein, “‘XBn’ Barrier Photodetectors for High Sensitivity and High Operating Temperature Infrared Sensors,” Proc. SPIE 6940 (2009) 69402U], where “X” stands for the contact layer, “B” stands for the barrier layer, and “n” stands for the active layer.
In the general XBn devices, the bandgaps of the contact layer and active layer may differ, while in the more specific nBn devices, the bandgap of the contact layer is generally the same as the bandgap of the active layer.
Additional background art includes U.S. Pat. No. 7,737,411, and U.S. Published Application Nos. 20070215900 and 20100295095.